1. Field of the Invention
The present invention is drawn to a telephone subscriber's subset and more particularly to the muting of signals in subscriber's subsets that utilize electronic tones. Such units involve audio amplifiers and speakers to replace the conventional electromechanical ringers usually found in subscriber's telephones.
2. Description of the Prior Art
With the advent of extensive use of electronic circuitry in the field of telephony, it has become common to utilize the receiver in telephone sets to reproduce incoming ringing signals as well as to perform the usual speech translating function. In such sets the receiver functions as a signaling reproducer while located on the hook switch. Obviously under certain conditions particularly that wherein the telephone is of a multi-line or key telephone type and connected to a plurality of lines, it is conceivable that a subscriber while utilizing the telephone to converse over one line might receive an incoming signal over another line with a resulting acoustic disturbance caused by the application of the incoming ringing signal to the receiver. Such an arrangement at best is highly unsatisfactory.
In other telephone substations a separate receiver transducer is utilized to reproduce incoming ringing signals, from that used to reproduce voice signals. However even in this situation incoming ringing signals may be disturbing to the subscriber utilizing the subset. Thus it is obvious that the problem of muting incoming ringing signals while a phone is being used is well known particularly in key telephone systems. A key telephone system as noted will often have the appearance of a number of lines and several of these lines may be connected to ring at the same telephone. It obviously then is an annoyance for the telephone to ring at full volume while the call is in progress. In the past, electromechanical ringers did not lend themselves to automatic off-hook muting. With the introduction of tone signaling and electronic amplifiers as the transmitters of this signaling, automatic off-hook muting becomes a practical feature to include in the key telephone.
Early attempts to solve the problem described above include that found in U.S. Pat. No. 2,972,021, to Bryant et al, which employed a complete redesign of the hook switch contacts and linkages. In this arrangement muting is performed by switching the signaling device in and out with dedicated hook switch contacts. Obviously in a situation such as this field conversion is not practical and special telephone instruments would have to be employed to provide the necessary muting.
Another approach to solve the same problem is shown in British Pat. No. 1,178,695, to Crooks et al, which utilizes a switching contact solely for muting but activation of this contact requires magnetic components mounted in the telephone handset. Again in this situation a special telephone is required with the utilization of unmodified key telephone sets being out of the question.
More recently the Western Electric 7A Key System has provided for signal muting by installing additional contacts on the telephone hook switch and adding a resistor in series with the amplifier speaker. When the telephone is not in use the extra hook switch contacts short the resistor and permit the entire audio output signals to reach the speaker. During a telephone conversation the resistance is included in the output circuit and some of the output signal is dissipated as heat in the resistor. This solution, while effective, requires new contacts on the hook switch. Again the additional contact requires a modification to the telephones basic construction and cannot be considered as a reasonable approach when electronic ringing is to be added to telephones already located in the field.
Accordingly it is the object of the present invention to provide a simple muting circuit which is able to sense an off-hook condition with one of the normal hook switch contacts already existing in standard key telephones. The present invention when added to an audio amplifier permits standard key telephones to be adapted for electronic ringing with automatic off-hook muting without modification to the telephones hook switch arrangement.